1. Field of the Invention
The present invention relates generally to an apparatus and method for transmitting/receiving a signal in a communication system, and more particularly to an apparatus and method for transmitting/receiving a downlink signal.
2. Description of the Related Art
Since limited resources, such as frequency, code and timeslot resources, are divided and used in multiple cells of a communication system with a cellular structure (hereinafter cellular communication system), Inter-Cell Interference (ICI) may occur. When the frequency resources are divided and used in the multiple cells of the cellular communication system, the ICI results in performance degradation. The frequency resources are reused to increase the overall capacity of the cellular communication system. Herein, the rate at which the same frequency resources can be reused is referred to as a “frequency reuse factor”. The frequency reuse factor is defined by the number of cells in which the same frequency resources are unused.
FIG. 1 illustrates the structure of the conventional cellular communication system using the frequency reuse factor of 1.
In FIG. 1, it is assumed that three cells of the cellular communication system, i.e., a first cell 110, a second cell 120 and a third cell 130, have a 3-sector structure, respectively. The first cell 110 has the 3-sector structure of a first sector 111, a second sector 113 and a third sector 115. The second cell 120 has the 3-sector structure of a first sector 121, a second sector 123 and a third sector 125. The third cell 130 has the 3-sector structure of a first sector 131, a second sector 133 and a third sector 135. Assuming that the cellular communication system uses the frequency reuse factor of 1, all the sectors of the first to third cells 110 to 130 use the same frequency resources, i.e., the same Frequency Allocation (FA) F1.
Since the same FA F1 is used in the cells and sectors, a channel state is poor in a cell boundary region. For example, a Carrier-to-Interference and Noise Ratio (CINR) is very low. Thus, the probability of occurrence of a reception error is high even though a signal is transmitted at the most robust Modulation and Coding Scheme (MCS) level supportable in an associated cell.
FIG. 2 illustrates the structure of the DL frame of the conventional communication system.
Referring to FIG. 2, the DL frame includes a preamble field 210, a Frame Control Header (FCH) field 220, a MAP field 230 and a burst field 240.
In the preamble field 210, a preamble signal is transmitted to acquire synchronization between a transmitter, for example, a Base Station (BS), and a receiver, for example, a Mobile Station (MS), and to identify the BS. In the FCH field 220, an FCH is transmitted and contains information about a modulation scheme applied to the MAP field 230 and the length of the MAP field 230. Herein, a size of the FCH field 220 is fixed, for example, to 24 bits. A preset fixed MCS level, for example, a Quadrature Phase Shift Keying (QPSK) 1/16 level, is applied to the FCH field 220.
A MAP message is transmitted in the MAP field 230 and contains position information about Downlink (DL) and Uplink (UL) burst fields, modulation scheme information, and allocation information about the DL and UL burst fields, i.e., information about whether the DL and UL burst fields are dedicatedly allocated to a specified MS or are commonly allocated to unspecified MSs.
The burst field 240 contains dedicated burst fields 243, 245, 247 and 249 dedicatedly allocated to specified MSs and a common burst field 241 commonly allocated to unspecified MSs. In the dedicated burst fields 243, 245, 247 and 249, dedicated burst data targeting the specified MSs, for example, traffic data and a dedicated control message, are transmitted. In the common burst field 241, common burst data targeting the unspecified MSs, for example, a common control message, is transmitted. In FIG. 2, it is assumed that the dedicated control message is transmitted only in the dedicated burst field 247 and the traffic data is transmitted in the remaining dedicated burst fields 243, 245 and 249.
As described above, the MAP message to be transmitted in the MAP field 230, the common control message to be transmitted in the common burst field 241 and the dedicated control message to be transmitted in the dedicated burst field 247 are mandatory information for communication between the BS and the MSs. Thus, the BS applies the most robust MCS level supportable therein, for example, a QPSK 1/12 level, to the MAP field 230, the common burst field 241 and the dedicated burst field 247. Therefore, all the MSs at the BS can receive the MAP message, the common control message and the dedicated control message without error.
MCS levels mapped to channel states of target MSs are applied to the dedicated burst fields 243, 245 and 249 targeting the MSs. That is, the BS sets the MCS levels to be applied to the dedicated burst fields 243, 245 and 249 on the basis of the channel states fed back from the target MSs, i.e., Channel Quality Indications (CQIs). When setting the MCS levels based on the CQIs, the BS can use a link curve of a short-term CQI or an average CQI over a preset time interval.
The most robust MCS level supportable in the BS is applied to the dedicated burst field for transmitting the dedicated control message among the MAP field, the common burst field and the dedicated burst field as described with reference to FIG. 2 such that all the MS at the BS can normally receive the MAP message, the common control message and the dedicated control message. As the most robust MCS level supportable in the BS is applied to transmit the MAP message, the common control message and the dedicated control message, an amount of resources for a traffic data transmission, i.e., a size of the dedicated burst fields, is reduced. The dedicated control message is a control message targeting only a specified MS. However, as the most robust MCS level supportable in the BS is applied to the dedicated control message, a significant waste of resources may occur.
When the frequency reuse factor of 1 is used as described with reference to FIG. 1, the ICI may be caused by a neighbor BS. The MAP message, the common control message and the dedicated control message may not be normally received due to the ICI even though the most robust MCS level supportable in an associated BS is applied. In particular, when there is a region where the MAP message and the common control message are not normally received, the region is a service shadow region at the associated BS. In the service shadow region, service provision itself is impossible and service stability of the overall communication system is degraded.